The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposed to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,118,071; 4,043,597; and 4,043,598 which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retoring zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate composition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
U.S. Pat. No. 4,043,598 discloses a method for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to a method disclosed in that patent, a plurality of vertically spaced apart voids of similar horizontal cross-section are initially excavated one above another within the retort site. A plurality of vertically spaced apart zones of unfragmented formation are temporarily left between the voids. Explosive is placed in each of the unfragmented zones and detonated, preferably in a single round, to explosively expand each unfragmented zone into the voids to form a fragmented mass. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
U.S. Patent Application Ser. No. 929,250 titled "METHOD FOR EXPLOSIVE EXPANSION TOWARD HORIZONTAL FREE FACES FOR FORMING AN IN SITU OIL SHALE RETORT" filed July 31, 1978, by the applicant describes a method for forming an in situ oil shale retort by expanding formation toward vertically spaced apart voids. Patent Application Ser. No. 929,250 is incorporated herein by this reference.
U.S. Pat. No. 4,043,596 discloses a method of forming an in situ oil shale retort in a subterranean oil shale deposit by excavating at least one vertically extending columnar void. The surface of the formation which defines the columnar void presents at least one free face which extends vertically through the subterranean oil shale deposit. A portion of the formation within the boundary of the in situ retort to be formed and which extends away from the free face is explosively expanded toward the columnar void in one or more segments. The expansion of the oil shale toward the columnar void fragments the oil shale, thereby distributing the void volume of the columnar void throughout the retort.
It is desirable to form an in situ retort with a reasonably uniformly distributed void fraction, or a fragmented mass of reasonably uniform permeability so that gas can flow reasonably uniformly through the fragmented mass during retorting operations. Techniques used for explosively expanding zones of unfragmented formation toward the horizontal free faces of formation adjacent the voids can control the uniformity of particle size or permeability of the fragmented mass. A fragmented mass having generally uniform permeability in horizontal planes across the fragmented mass avoids bypassing portions of the fragmented mass by retorting gas as can occur if there is gas channeling through the mass owing to non-uniform permeability.
In a technique for forming an in situ oil shale retort by explosive expansion of unfragmented formation toward a free face, inefficient distribution of explosive can occur due to use of blastholes extending perpendicular to such a free face. Because the explosive is in columns perpendicular to the free face, the energy from detonation of such explosive is less effeciently utilized than if the columns of explosive were parallel to the free face.
Alternatively, when columns of explosive are provided which are parallel to a free face, the explosive may not be distributed within the formation to the extent desirable because only a limited number of blastholes are used due to the economic considerations or the like.
It is, therefore, desirable to provide a method of forming an in situ oil shale retort which provides for improved efficiency in explosively expanding formation toward a free face.